1. Field of the Invention
The present invention relates to a single side-sealed metal vapor discharge lamp and, more particularly, to a small metal halide lamp etc. having a sealed section at a single end only.
2. Description of the Related Art
Recently, a metal vapor discharge lamp also called a high intensity discharge (HID) lamp has increasingly been employed to illuminate the interior of a house, such as a store. A metal halide lamp in particular has widely been adopted to illuminate merchandise in the store, preferably in terms of its high efficacy and its good color rendering properties.
In such indoor illumination installation, it is necessary to make the lamp's size small and the lamp fitting compact.
If, in order to achieve the aforementioned object, a sealed section is to be provided at each end of a discharge tube for a conventional inner arc tube, then more manufacturing steps are required in the formation of the inner arc tube, and the size of the resultant inner arc tube is increased with a size increase in its sealed sections and there is a greater loss of heat coming from the discharge tube.
In order to obtain a compact lamp, it is advantageous to, as disclosed in Japanese Patent Disclosure (KOKAI) 60-232662, form a pinch-sealed section at one end of the discharge tube and a discharge region defining envelope at the other end of the discharge tube to provide a single side-sealed structure.
The single side-sealed type inner arc tube can be made more compact than the inner arc tube whose discharge tube is sealed at both the ends. It is thus possible to decrease a heat loss, because the inner arc tube of interest is of a single side-sealed type, and to improve the light emitting efficiency. Further, it does not take a lot of time to perform a sealing step because the sealed section is of a single type.
In the single side-sealed inner arc tube, a pair of metal foils such as molybdenum foils are sealed in the single side-sealed section and a pair of metal foils are connected to the corresponding pair of electrodes. The electrodes are each composed of a rod connected at one end to the corresponding metal foil and extending at the other end into the discharge region and a coil mounted on the other end portion of the rod. The electrode rod and electrode coil are formed of a tungsten wire or a thoriated tungsten (ThO.sub.2 -W).
In order to enhance the emission efficiency, the single side-sealed compact metal halide lamp is lighted at a high lamp load satisfying an equation WL/S=20 to 70 where WL (watt) and S (cm.sup.2) represent the input power of the lamp and the inner surface area, defined in the discharge region, respectively.
Upon the lighting of the lamp at a high load, a metal halide acts upon the electrode rod and, during the life of the lamp, the electrode rod is attacked by the halogen and narrowed down. As a result, tungsten (W) or ThO.sub.2 -W in the electrode rod is sputtered onto the discharge tube wall, causing blackening on the tube wall and sometimes a breakage of the electrode rod.
If the electrode rod is formed of tungsten (W) or thoriated tungsten (ThO.sub.2 -W), no better connection can be obtained between the rod and the metal foil because W or ThO.sub.2 -W is high in the melting point. It takes lots of time in the welding operation.
Under study is the way of preparing an electrode rod made of a pure rhenium metal or a rhenium/tungsten alloy which are excellent in resistance to halogen and low in the melting point or coating the surface of the electrode rod of tungsten with a pure rhenium metal or a rhenium/tungsten alloy.
If the electrode rod is formed of a pure rhenium metal or a rhenium/tungsten alloy which are excellent in resistance to halogen and low in melting point or the electrode rod made of tungsten is coated with a pure rhenium metal or a rhenium/tungsten alloy, it is desired that the electrode coil be made up of W or ThO.sub.2 -W. In the case where the electrode coil is formed of a pure rhenium metal or a rhenium/tungsten alloy, since the coil material is lower in melting point than the tungsten in spite of its excellent resistance to halogen, the material of which the electrode coil is made is sputtered from an arc spot onto the discharge tube wall. As a result, blackening on the tube wall progresses for a short period of time, causing a greater fall in lumen maintenance factor. As, in particular, this type of lamp is smaller in the surface area of an inner arc tube, blackening on the tube wall rapidly progresses upon the sputtering of the electrode coil material even if being smaller in quantity, so that the lumen maintenance factor drops to a greater extent.
From this it may be considered that the electrode rod is formed of a pure rhenium metal or a rhenium/tungsten alloy or the tungsten rod is coated with the rhenium metal or the rhenium/tungsten alloy and that the electrode coil is formed of W or ThO.sub.2 -W.
In the case where the electrode is composed of a rod and a coil, if the forward end of the electrode rod extends from the end of the coil toward the discharge region, an arc spot is generated at the forward end portion of the rod. Since the rod is formed of a pure rhenium metal or a rhenium/tungsten alloy as set out above or the tungsten rod is covered with the pure rhenium metal or a rhenium/tungsten alloy, it melts at a lower temperature level and the material of which the electrode rod is made is scattered onto the tube wall during the life of the lamp, causing blackening on the tube wall. As a result, the lumen maintenance factor is lowered.
If the size of the electrode coil is too large, the heat capacity becomes greater and hence a temperature rise is hard to produce in the electrode coil at a time of starting. As a result, no steady arc is obtained and the startability is lowered.